1. FIELD OF THE INVENTION
This invention relates to a method and apparatus for the detection of ionized species in a gas carrier, and more particularly to an improved radiation source for producing ionizing radiation in a photoionization detector and to an improved method of irradiating an ionization chamber to create more efficient ionization of at least some of the species.
2. DESCRIPTION OF THE PRIOR ART
Photoionization detectors are available for use with a gas chromatograph column. Ultraviolet radiation is concentrated in an ionization chamber which receives the output from the column. This output consists of a carrier gas and the chemical species being studied. The photon energy of the radiation is at a level designed to selectively ionize the species to be detected rather than the carrier gas. Such ionization is detected by an electrical circuit connected to electrodes in the chamber to provide both a direct read-out and to drive a chart recorder.
In gas chromatography, the carrier gas flows continuously through the device, passing first through the chromatograph column and then through the ionization chamber. A gaseous or liquid diluent, containing the ionizable chemical species under study, is introduced into the stream of carrier gas to be carried through the column to elute at different rates. This causes a time separation of the species so that they can be identified individually (or at least in small groups) in the photoionization detector. When displayed on a chart recorder, the time separation results in the species appearing as a series of peaks whose arrival time is a function of the time taken for that particular species to elute through the column before being ionized. By comparison with known standards, the species can be identified and their quantity measured by using the area under the peak displayed on the chart recorder.
The radiation used in a photoionization detector should be of high enough energy to ionize the chemical species to be detected, but not so high as to discernably ionize the carrier gas or any other species present which it is not desired to detect. Generally speaking the radiation used is ultraviolet radiation in the range 1000 A.U. to 2000 A.U. Such radiation will not ionize any of the permanent air gases, nor will it ionize water vapour. The radiation is quickly absorbed in air so that to be useful it is used in a vacuum or an atmosphere of inert gas. Because of this it is commonly referred to as vacuum ultraviolet radiation.
Presently, the radiation source is commonly a gas discharge tube maintained at low pressure and having a crystal window of an appropriate transmissive material to provide an exit for the ultraviolet radiation. The discharge or excitation is produced by maintaining a constant high potential across two metal electrodes within the tube and in contact with the gas.
In a discharge tube of the above type, complicated tube designs have to be used in order to prevent electrode erosion caused by ion impact. This problem is referred to as "sputtering" and can result in electrode metal being deposited upon the inner surface of the crystal window with resulting reduction in transmission through the window.
U.S. Pat. No. 3,933,432 to Driscoll issued on Jan. 20, 1976 and is an example of a prior art structure which is designed to minimize sputtering. In this structure, the gas discharge has been mainly confined to a central capillary within the discharge tube by constraining the flow of ions as they move from one electrode to another. This design creates what is effectively a "point source" of vacuum UV radiation, originating from the small cross section of the capillary. As a result, the distribution of radiative flux across the diameter of the ion chamber in any plane perpendicular to the direction of travel of the radiation entering the chamber is non-uniform. There is a high concentration at the centre and a low concentration at the periphery. Apart from the reduced ionization due to limited total flux, such a design will exhibit strong "quenching" effects whenever a trace of oxygen is present in the chamber. Such quenching occurs when an electron, generated as a result of photoionization, becomes attached to an oxygen atom, due to the high electron affinity of oxygen. The resulting negative ion has a mobility far lower than the original electron, and is far more likely to recombine with a positively charged ion before it can be detected. Quenching is therefore a severe problem in such a device.
Discharge tubes have also been built with metal electrodes mounted externally of an all-glass tube in a capacitive relationship. For example, U.S. Pat. No. 3,996,272, to Young shows a structure having one electrode inserted into a hollow re-entrant capillary running up the axis of a cylindrical discharge cavity, and a second electrode formed as a metal cylinder wrapped around the outside of the tube. The resulting coaxial electrode configuration functions as a capacitor and is driven by a supply of radio frequency power. Tubes of this type are relatively difficult and expensive to make and, due to obstruction by the re-entrant capillary, do not have a radially uniform output intensity at the window.
In accordance with one aspect of the present invention, a source of photoionization is provided for use in a detector. The source is in the form of an electrodeless gas discharge tube which is excited inductively by the use of a coil connected to a radio frequency oscillating circuit. The inductance coil is provided around the outside of the tube and is tuned to excite radiatively, the gas in the tube so that ultraviolet radiation is emitted through a window in the end of the tube.
In another of its aspects, the invention provides a photoionization detector using the aforesaid source of photoionization in which the window also forms an end wall of an ionization chamber so that the ultraviolet radiation can travel into the ionization chamber. Electrodes are positioned such that the ultraviolet radiation will not impinge on the cathode.
The inductive coil excites the gas in the tube across its whole cross-sectional area and thus produces a uniform distribution across this area which corresponds in size both to the window and to the ionization chamber. As a result the chamber is irradiated substantially uniformly with respect to a plane extending transversely of the axis of the tube and hence of the coil.
These and other aspects of the invention will be better understood with reference to the following description taken in combination with the drawings.